1. Geant4 v9.4 Primary Particle Makoto Asai (SLAC) Geant4 Tutorial Course

2. Contents G4VUserPrimaryGeneratorAction Primary vertex and primary particle Built-in primary particle generators –Particle gun –Interfaces to HEPEVT and HEPMC –General particle source More on particle gun Primary Particle - M.Asai (SLAC)2

3. Geant4 v9.4 Primary particle generation

4. User classes Initialization classes –Use G4RunManager::SetUserInitialization() to define. –Invoked at the initialization G4VUserDetectorConstruction G4VUserPhysicsList Action classes –Use G4RunManager::SetUserAction() to define. –Invoked during an event loop G4VUserPrimaryGeneratorAction G4UserRunAction G4UserEventAction G4UserStackingAction G4UserTrackingAction G4UserSteppingAction main() –Geant4 does not provide main(). Note : classes written in red are mandatory. Primary Particle - M.Asai (SLAC)4

5. G4VUserPrimaryGeneratorAction This class is one of mandatory user classes to control the generation of primaries. –This class itself should NOT generate primaries but invoke GeneratePrimaryVertex() method of primary generator(s) to make primaries. Constructor –Instantiate primary generator(s) –Set default values to it(them) GeneratePrimaries() method –Randomize particle-by-particle value(s) –Set these values to primary generator(s) Never use hard-coded UI commands –Invoke GeneratePrimaryVertex() method of primary generator(s) Primary Particle - M.Asai (SLAC)5

6. Geant4 v9.4 Primary vertex and primary particle

7. Primary vertices and particles Primary vertices and primary particles are stored in G4Event in advance to processing an event. –G4PrimaryVertex and G4PrimaryParticle classes These classes don ’ t have any dependency to G4ParticleDefinition nor G4Track. –Capability of bookkeeping decay chains Primary particles may not necessarily be particles which can be tracked by Geant4. Geant4 provides some concrete implementations of G4VPrimaryGenerator. –G4ParticleGun –G4HEPEvtInterface, G4HEPMCInterface –G4GeneralParticleSource Primary Particle - M.Asai (SLAC)7

8. Pre-assigned decay products Physics generator can assign a decay channel for each individual particle separately, while in Geant4 you cannot specify a decay channel for each particle. –Decay chain can be “pre-assigned”. A parent particle in the form of G4Track object travels in the detector, bringing “pre- assigned” decay daughters as objects of G4DynamicParticle. –When the parent track comes to the decay point, pre-assigned daughters become to secondary tracks, instead of randomly selecting a decay channel defined to the particle type. Decay time of the parent can be pre-assigned as well. Primary Particle - M.Asai (SLAC)8 D0D0 --  K-K- ++  B-B- G4PrimaryParticle B-B- G4Track D0D0 --  K-K- ++  pre-assigned decay products K-K- ++  D0D0 --  B-B- K-K- ++  D0D0

9. Geant4 v9.4 Built-in primary particle generators

10. G4ParticleGun Concrete implementations of G4VPrimaryGenerator –A good example for experiment-specific primary generator implementation It shoots one primary particle of a certain energy from a certain point at a certain time to a certain direction. –Various set methods are available –Intercoms commands are also available for setting initial values One of most frequently asked questions is : I want “ particle shotgun ”, “ particle machinegun ”, etc. Instead of implementing such a fancy weapon, in your implementation of UserPrimaryGeneratorAction, you can –Shoot random numbers in arbitrary distribution –Use set methods of G4ParticleGun –Use G4ParticleGun as many times as you want –Use any other primary generators as many times as you want to make overlapping events Primary Particle - M.Asai (SLAC)10

11. G4VUserPrimaryGeneratorAction void T01PrimaryGeneratorAction:: GeneratePrimaries(G4Event* anEvent) { G4ParticleDefinition* particle; G4int i = (int)(5.*G4UniformRand()); switch(i) { case 0: particle = positron; break;... } particleGun->SetParticleDefinition(particle); G4double pp = momentum+(G4UniformRand()-0.5)*sigmaMomentum; G4double mass = particle->GetPDGMass(); G4double Ekin = sqrt(pp*pp+mass*mass)-mass; particleGun->SetParticleEnergy(Ekin); G4double angle = (G4UniformRand()-0.5)*sigmaAngle; particleGun->SetParticleMomentumDirection (G4ThreeVector(sin(angle),0.,cos(angle))); particleGun->GeneratePrimaryVertex(anEvent); } You can repeat this for generating more than one primary particles. Primary Particle - M.Asai (SLAC)11

12. Interfaces to HEPEvt and HepMC Concrete implementations of G4VPrimaryGenerator –A good example for experiment-specific primary generator implementation G4HEPEvtInterface –Suitable to /HEPEVT/ common block, which many of (FORTRAN) HEP physics generators are compliant to. –ASCII file input G4HepMCInterface –An interface to HepMC class, which a few new (C++) HEP physics generators are compliant to. –ASCII file input or direct linking to a generator through HepMC. Primary Particle - M.Asai (SLAC)12

13. G4GeneralParticleSource A concrete implementation of G4VPrimaryGenerator –Suitable especially to space applications MyPrimaryGeneratorAction:: MyPrimaryGeneratorAction() { generator = new G4GeneralParticleSource; } void MyPrimaryGeneratorAction:: GeneratePrimaries(G4Event* anEvent) { generator->GeneratePrimaryVertex(anEvent); } Detailed description http://reat.space.qinetiq.com/gps/ Primary Particle - M.Asai (SLAC)13

14. G4GeneralParticleSource Primary vertex can be randomly chosen on the surface of a certain volume. Momentum direction and kinetic energy of the primary particle can also be randomized. Distribution could be set by UI commands. Capability of event biasing (variance reduction). –By enhancing particle type, distribution of vertex point, energy and/or direction Primary Particle - M.Asai (SLAC)14 Square plane, cosine-law direction, linear energy Spherical surface, isotropic radiation, black-body energy Cylindrical surface, cosine-law radiation, Cosmic diffuse energy Spherical volume with z biasing, isotropic radiation with theta and phi biasing, integral arbitrary point-wise energy distribution with linear interpolation.

15. Geant4 v9.4 Particle gun

16. Particle Gun vs. General Particle Source Particle Gun –Simple and naïve –Shoot one track at a time –Easy to handle. Use set methods to alternate track-by-track or event-by- event values. General Particle Source –Powerful –Controlled by UI commands. Almost impossible to control through set methods –Capability of shooting particles from a surface of a volume. –Capability of randomizing kinetic energy, position and/or direction following a user-specified distribution (histogram). Primary Particle - M.Asai (SLAC)16 If you need to shoot primary particles from a surface of a volume, either outward or inward, GPS is the choice. If you need a complicated distribution, not flat or simple Gaussian, GPS is the choice. Otherwise, use Particle Gun.

17. What to do and where to do In the constructor of your UserPrimaryGeneratorAction –Instantiate G4ParticleGun –Set default values by set methods of G4ParticleGun Particle type, kinetic energy, position and direction In your macro file or from your interactive terminal session –Set values for a run Particle type, kinetic energy, position and direction In the GeneratePrimaries() method of your UserPrimaryGeneratorAction –Shoot random number(s) and prepare track-by-track or event-by-event values Kinetic energy, position and direction –Use set methods of G4ParticleGun to set such values –Then invoke GeneratePrimaryVertex() method of G4ParticleGun –If you need more than one primary tracks per event, loop over randomization and GeneratePrimaryVertex(). examples/extended/analysis/A01/src/A01PrimaryGeneratorAction.cc is a good example to start with. Primary Particle - M.Asai (SLAC)17

18. G4VUserPrimaryGeneratorAction void A01PrimaryGeneratorAction:: GeneratePrimaries(G4Event* anEvent) { G4ParticleDefinition* particle; G4int i = (int)(5.*G4UniformRand()); switch(i) { case 0: particle = positron; break;... } particleGun->SetParticleDefinition(particle); G4double pp = momentum+(G4UniformRand()-0.5)*sigmaMomentum; G4double mass = particle->GetPDGMass(); G4double Ekin = sqrt(pp*pp+mass*mass)-mass; particleGun->SetParticleEnergy(Ekin); G4double angle = (G4UniformRand()-0.5)*sigmaAngle; particleGun->SetParticleMomentumDirection (G4ThreeVector(sin(angle),0.,cos(angle))); particleGun->GeneratePrimaryVertex(anEvent); } You can repeat this for generating more than one primary particles. Primary Particle - M.Asai (SLAC)18